HIGH IMPACT WEATHER AFFECTING ZIMBABWE by 8KY1ZJPu

VIEWS: 2 PAGES: 16

									                   By
Tavonga Gilbert Diwah
    High-impact weather and climate related
    events that have severe consequences for
    local populations in terms of livelihood,
    food, water security and health impacts
    in Zimbabwe include:
    •   extreme rainfall events,
    •   late onset or late withdrawal of the rainy seasons,
    •   tropical cyclones,
    •   prolonged drought periods and extensive mid
        season dry spells.
   Causes
    • tropical cyclones,
    • convective systems and
    • middle level stratiform clouds.

   Convective rainfall is one of the most damaging
    meteorological phenomena as it can result in massive
    amounts of rainfall occurring in short periods of time
    leading to flash floods. Improvements in satellite data over
    recent years, notably the increased resolution of images,
    increased range of satellite derived products and shorter
    time intervals at which the data is received, has improved
    the monitoring of the development and movement of
    convective systems over relatively small areas.
   The Severe Weather Forecasting Demonstration
    Project (SWFDP) currently being rolled out
    across the SADC region encouraged member
    countries to make use satellite products in
    forecasting severe weather systems affecting
    their individual countries. Undoubtedly, much of
    the severe weather experienced in Zimbabwe is
    associated convective systems and rainfall
    derived thereof.
   Most of the severe weather events reported in
    Zimbabwe during the past season were heavy
    rainfall events.
 Diagnostic     examination     of    ECMWF
  composite synoptic-scale features, indicates
  that lower northerly and upper easterly flow
  is     enhanced       during   wet    spells,
  corresponding with a tropical low over
  Zambia and an anticyclone off the south-
  east coast of Africa.
 Dry spells exhibit an increase in mid-
  latitude cyclones off the south-west coast of
  Africa    and     tropical   cyclones   near
  Madagascar
 . Mid-tropospheric troughs are located over
 the east and west coasts of Africa near 25°S
 and serve to increase anticyclonic vorticity
 over Zimbabwe in dry spells. Cyclonic
 vorticity and low geopotential heights occur
 on two or three days in the wet spell,
 coincident with intense cumulus convection
 and high rain rates. The eddy covariances of
 zonal wind and specific humidity are
 dominated by eastward fluxes in dry spells.
 The eddy fluxes are shifted 15-20° longitude
 eastward in the wet spells.
   The 2002/03 rainfall season came against the backdrop of a
    weak El-Nino event in the eastern equatorial Pacific, which
    only petered off during early 2003. As a result, the 2002/03-
    rainfall season was largely characterized by prolonged dry
    spells and patchy rains in some parts of the country.
    However, the latter part of the second half of the season
    (January to March) saw the development of various rain-
    bearing weather systems which brought a lot of rainfall
    leading to flooding in Mashonaland Central, southern
    sections of Manicaland and Masvingo provinces.

   The average cumulative rainfall that occurred across the
    country since the beginning of the season to March 26, 2003
    was 593.8 mm. This is below the national long-term mean of
    662.3 mm by 68.5 mm but higher than that of the 2001/02
    season.
    The effective rainfall season started in the
    last 11 days of October for most parts of
    the country with the exception of most of
    Mashonaland Central and the central
    districts of Mashonaland East, which
    remained dry until the beginning of
    November. The last 20 days of November
    were generally dry across the whole
    country.
.
   Dry spells were also evident in the first and third
    dekads of December, the first, second and third
    dekads of January, first and second dekads of
    February in different parts of the country. The
    highest frequency of dry spells lasting for 10 or more
    days occurred in Matabeleland South followed by
    Manicaland and southern parts of Midlands. The
    longest dry spell of thirty-two (32) consecutive days
    occurred in Beitbridge during the first half of the
    season. These dry spells impacted negatively on
    crop development, particularly the early-planted
    crop that endured tong periods of moisture stress.
    There were, however, no evident dry spells of 10 or
    more days in the major maize producing provinces of
    Mashonaland West and Mashonaland Central since
    December 2002
   The tropical cyclone season in the South-West
    Indian Ocean Basin stretches from November to
    April, the onset varying across the geographical
    positions. In Zimbabwe, the season normally
    starts in December and ends in March (4
    months). However, these cyclones have
    influenced the country outside the period above.
    Also, only tropical cyclones in the Mozambique
    Channel have a direct bearing on Zimbabwe,
    with either floods or in-season prolonged dry
    spells. Either way, the results have negative
    impacts on agriculture, the mainstay of the
    country’s economy.
   A low pressure area
    developed in the
    Mozambique Channel
    on the 21st of February
    and intensified into a
    tropical cyclone, Japhet,
    by 28 February. The
    cyclone then started
    moving SSW at 5 knots
    (9km/hr) with winds of
    65 to 70 knots (120 to
    130km/hr). The central
    surface pressure was
    about 965hPa.
   By the time it reached
    southeast Zimbabwe (near
    30 E and 20 S) on the 4th,
    the maximum winds had
    been reduced to between
    60 and 80km/hr. It however,
    caused a lot of rainfall over
    many parts of Zimbabwe. It
    continued to weaken and
    by 12:00 GMT on 5 March,
    the system had been
    downgraded to just being
    an ordinary low-pressure           IR image of TC Japhet centered over central
                                              Mozambique on the 3rd of March
    system.                                          (06:30 GMT)

   The depression
    nevertheless caused
    heavy falls in the south
    and eastern areas of
    Zimbabwe. An example
    is Masvingo, which
    recorded 204mm of rain
    in the 24 hours from 6 to
    7 March. Rupike, also in
    the southeast, recorded
    an amount almost seven
    times its March mean in
    the 4 days from 3 to 7
    March.
 Zimbabwe, a  largely agricultural country,
 has experienced almost successive
 drought/flooding periods, which in the
 past two decades have become more
 frequent and increased intensity. The
 food insecurity that occurred in the
 country in the past years was a
 cumulative consequence of recurrent
 disasters, which actually eroded the
 population's coping mechanisms.
 Drought is the most
 economic damaging
 phenomena in the
 country with more
 than 2.5 Billion
 dollars loss since
 1982. The following
 graph shows the top
 two economic losses
 since 1982.

								
To top